This invention relates to the calibration of flow meters.
The increasing need of companies such as water utilities to comply with BS5750 (ISO900I) is forcing them to perform regular routine checks on all their equipment. In the case of a free-standing or hand held instrument this is not difficult; it is sent away to a calibration house annually for a calibration check and issue of an appropriate certificate. The case of an installed flowmeter is not so easy. The transmitter can be checked with a simulator which can itself be checked by the manufacturer to BS5750. The sensor however, at present, must be removed from the pipeline (and possibly from under the ground) and returned to a test facility for wet calibration checking.
Furthermore, in some markets such as the U.S.A. and Canada, there is some reluctance to accept electromagnetic flow meters as measuring devices since currently they cannot be calibrated in situ. There is a preference for Venturi tubes, the performance of which can be measured directly.
It is an object of at least the preferred embodiments of the present invention to avoid the foregoing problem and perceived disadvantage of electromagnetic flowmeters.
In one aspect, the invention provides a method of in situ testing of an electromagnetic flowmeter comprising measuring at least two electrical parameters (preferably substantially independent parameters) of the sensor coil circuit, comparing the measured parameters to stored parameters, and determining whether the calibration of the flowmeter is accurate based on the results of the comparison. This may allow calibration drift to be assessed reliably.
The at least two electrical parameters preferably include a measure of ohmic resistance and a measure of inductance of the sensor coil; these parameters together may give reliable characterisation of the coil. Impedance and other parameters are preferably measured according to one of the aspects below.
Optionally a measure of losses in the coil is used in said determining, for example in addition to ohmic resistance and inductance; this can further improve detection of of changes in the coil properties.
Preferably, the stored parameters comprise parameters measured from the same flowmeter at an earlier time. Storing parameters for an individual flowmeter, for example during manufacture or initial calibration, may enable a more accurate determination of changes in meter properties that is less sensitive to manufacturing tolerances.
However, the stored parameters may comprise parameters defined for a batch of the flowmeters, for example on the basis of mean or predicted parameters for a model or a batch of meters of a particular model; this reduces the amount of data that need be stored.
Determination of the parameters preferably includes measuring an exponentially-changing current (or related variable) through a coil and deriving a time-constant therefrom, for example according to one of the following aspects.
According to another aspect of the invention there is provided a method of in situ testing of an electromagnetic flowmeter comprising determining the ohmic resistance of an energising coil thereof, measuring an exponentially-changing current across the coil, or a variable related thereto, deriving a time constant therefrom and determining the inductance of the coil from the time constant and the ohmic resistance.
In an apparatus aspect the invention provides electromagnetic flowmeter testing apparatus comprising means for determining the ohmic resistance of an energising coil of the flowmeter, means for providing an exponentially-changing current in the coil, means for measuring the current or a variable related thereto, means for deriving a time constant therefrom and means for determining the inductance of the coil from the time constant and the resistance.
The exponentially changing current may be produced by effecting a step change in a voltage applied to the coil.
The step change may be produced by removing the applied voltage.
Preferably the ohmic resistance is ratiometrically determined by reference to a resistor of known value in series with the coil.
The invention additionally may include measuring one or more of the following:
the isolation-to-ground impedance of the sensor coil;
the isolation-to-ground impedance of a sensor electrode of the flowmeter;
the eddy current resistance or loss component of the flowmeter.
Thus the method may include repeating the measurement of the exponentially-changing current or a variable related thereto later in time than the first measurement, and employing the two measurements and the times thereof either to derive an eddy current resistance of the flow meter, or to correct the determined value of the coil inductance for the effects of the resistance.
The method then preferably comprises measuring the elapsed times at which the measurements are taken after the step voltage change is effected, and utilising the elapsed times in the derivation of correction resistance.
In a further aspect the invention thus comprises a method in-situ testing a flowmeter comprising measuring an exponentially changing current in an energising coil of the flowmeter or a variable related thereto at two known time intervals from a datum and deriving from the measurements and time intervals an eddy current resistance of the flowmeter.
In a yet further aspect the invention provides a method of testing an electromagnetic flowmeter comprising applying to a component thereof an AC test signal of a frequency equal to that of a drive signal applied to an energising coil of the flowmeter during normal operation thereof, the test signal producing in the circuit a response characteristic of an impedance of the component and measuring the response by means of sensing means of the flowmeter which during normal operation senses a signal characteristic of fluid flow through the flowmeter.
In yet another aspect there is provided electromagnetic flowmeter testing apparatus comprising means for applying to a component thereof an AC test signal of a frequency equal to that of a drive signal applied to an energizing coil of the flowmeter during normal operation thereof, so as to produce in the circuit a response characteristic of an impedance of the component and means for supplying the response to sensing means of the flowmeter which during normal operation senses a signal characteristic of fluid flow through the flowmeter, whereby the sensing means measures the response.
The component may be an electrode and the impedance may be the isolation impedance thereof.
Alternatively or in addition the component may be the energising coil of the flowmeter, and the impedance in the isolation impedance of the coil.
The AC test signal may be obtained from the drive signal.
The test apparatus of the invention may be packed in portable form for use on site to test installed flowmeters.
The apparatus also may be incorporated into the flowmeter (which term includes its immediately adjacent drive and transmitter circuitry) and may be configured to operate automatically at intervals based either on time or on the total volume of fluid which has passed through the meter. Thus a self-testing and self-calibrating meter can be provided.
The invention also provides apparatus for testing an electromagnetic flowmeter comprising means for measuring at least two electrical parameters of the sensor coil circuit, means storing predetermined values for the at least two electrical parameters, means for comparing the stored parameters to the measured parameters, and means for outputting an indication of whether the flowmeter is within calibration based on the results of the comparison. The output may be visual or audible, for example at the time of testing, or the results may be output as data, for later evaluation.
The apparatus is preferably arranged to carry out testing according to one or more of the above method aspects, and may incorporate further features of the above apparatus aspects.
The invention now will be described merely by way of example with reference to the accompanying drawings, wherein: